1. Field of the Invention
The present invention relates to an improved process for the removal of silicon, carbon, manganese and sulphur from molten high carbon farrous metal and an apparatus therefor. The invention particularly relates to an improved process for the pre-treatment of the molten hot metal, customarily produced in blast furnace, from which silicon, carbon, manganese and sulphur are removed all together in a reaction vessel such as a customary blast furnace or a transfer ladle used in a steel plant. In the process of the present invention, no re-lading or removal of the slag is involved till the desired removal of silicon, carbon, manganese and sulphur-all together from the hot molten high carbon farrous metal-is accomplished. Thus, the process of the present invention is operationally very simple and saves a significant amount of energy.
The iron oxides such as iron ores blue dust, mill scale, sinter returns etc. can be used. The fluxes such as lime, flourspar, and rice husk and the metallic magnesium either with or without aluminium may be used.
2. Description of the Related Art
In order to convert the hot metal from a blast furnace or the molten cast iron metal from cupola (both these two metals are hereinafter referred to as hot metal) into steely the carbon, silicon and manganese present in the hot metal have to be removed. Similarly, sulphur content of the hot metal has to be decreased so that steels of a better quality or with a higher value, can be produced.
The removal of carbon, silicon and manganese from the hot metal in a steel plant is usually accomplished in a steel making unit, such as: basic oxygen furnace, open hearth furnace, etc. If the process of pre-treatment of hot metal is adapted, the metallurgical work to be done to remove carbon, silicon, and manganese in the steel making units is partially accomplished during the pre-treatment and also the sulphur is removed to an acceptable level before the hot metal is processed in a steel making units This
In the pre-treatment processes used thus far, the removal of carbon, silicon and manganese is accomplished through the addition of mill scale and/or is followed by oxygen lancing of the hot metal kept in a ladle or a reaction vessel. Besides in such processes, the removal of the silicon and of the sulphur is accomplished in separate stages or vessels with successive slag removal each time, producing a stoppage in the movement of the ladle or the hot metal.
Also, in such pre-treatment processes, the amount of silicon that can be removed is rather limited (about a maximum of 0.2%). On the other hand, the oxygen lancing which can produce higher desiliconisation or decarbonisation, also results in a loss of iron into the slag phase as iron oxide and also generates significant amount of fumes.
At present, the removal of sulphur from the hot metal is accomplished, in most instances, by introducing magnesium in the molten hot metal since magnesium is the most efficient reagent for removing sulphur.
The solubility of magnesium in the molten iron is limited. Besides, the recovery of magnesium and efficiency of the treatment when pure or almost pure magnesium is used, is very low, because before the magnesium can participate in the desulphurisation reactions it floats on the surface of the molten metal due to its low density. Also, being combustible, it immediately burns.
If the magnesium is plunged into the hot metal, it vapourises as soon as it is brought in contact with the hot metal. The vapourisation leads to a very large increase in the volume of magnesium and, as a result, the plunging treatment with magnesium results in an explosively violent reaction, wherein almost the entire volume of hot metal is ejected out of the reaction vessel at high velocity. Such a method of magnesium treatment is unsafe and is highly hazardous. Besides, this treatment is not reliable as the contact time between magnesium and the hot metal is very short. Hence the quantity of sulphur removed is practically negligible.
In order to avoid such a violent reaction, magnesium could be introduced as an alloy in the hot metal either as Mag-Coke, or Ni--Mg Alloy, or as a Fe--Si--Mg alloy. In such an instance, though the reaction is somewhat controlled making it non-vioment, the use of Ni--Mg or Fe--Si--Mg to remove sulphur is prohibitively expensive and also the plunging of the Mg-coke makes the plunging system very big and costly.
However, sulphur can be removed from the hot metal if steady and continuous source of magnesium vapour is maintained in the hot metal. At presents in one of the widely used techniques of desulphurisation, such a source of magnesium coated with less reactive materials, is suspended in a carrier gas such as nitrogen and is then injected into the hot metal with the help of a specially designed dispenser to deliver the coated granules or powder inside the hot metal. But, the technique has some inherent problems, such as availability of the high purity nitrogen; and the loss of unreacted magnesium as its vapour would tend to react chemically with nitrogen and is also physically carried up along with nitrogen as it bubbles up, without fully reacting with the sulphur in the hot metal. This causes lower efficiency of magnesium utilisation for sulphur removal.
Such a source of magnesium vapour can also be produced by impregnating magnesium into a pre-heated porous refractory material by dipping and soaking the material in molten magnesium. Subsequently, the refractory material is plunged into the molten iron to release the magnesium soaked in the refractory material.
Alternatively, the magnesium can be contained in a special chamber at the bottom of a special reaction vessel called `converter`. The magnesium is inserted in the chamber of the converter when it is empty. After, the converter is filled with hot metal, the magnesium melts and is released through the hot metal producing desulphurisation. This needs expensive capital equipment.
In yet another method, the magnesium can be contained in a plunger head with its open end threaded and finally closed with a threaded plug and with its wall containing perforations, through which the magnesium vapour escapes after the plunger is dipped and held in the hot metal. This needs a new plunger head for every treatment.
In yet another method, the open end of the plunger head is closed with a disc shaped piece made using refractory material. The plunger head is lowered independently of the ladle cover and the head is made to contact the bottom of the ladle and kept pressed against the bottoms in physical contact with it. Even though this plunging operation process is simple the plunger head used in such a process is expensive, due to the expensive material and fabrication required for its making. Besides the plunger head often breaks before the reaction is complete. Also, in such plunging operations, the plunger head breaks after every treatment for several reasons. First, the thermal expansion of the plunger rod and head produces mechanical load on the plunger head when kept pressed against the bottom of the ladle, during plunging. Secondly, the breakage also occurs since the turbulence in the molten iron produced by the expanding magnesium vapour, shakes the ladle and transmits the mechanical load to the plunger head, because it is kept pressed in physical contact with the bottom of the ladle. Since the plunger can not be reused, and is itself of desulphurisation by magnesium treatment.
A primary object of the present is to propose an improved process for the removal of silicon, manganese and sulphur from molten high carbon ferrous metal in a single reaction vessel.
Another object of the present invention is to propose an improved process for the removal of silicon, manganese and sulphur from molten high carbon ferrous metal in a single step effectively and economically without the fume generation or a significant drop in temperature.
Yet another object of the present invention is to propose an improved process for the removal of silicon, carbon, manganese and sulphur all together from molten high carbon ferrous metal in a single step, wherein the reversion of the sulphur removed earlier from the hot metal back into hot metal from the sulphur-rich slag produced is negligible, even after its prolonged holding in contact with the slag or even after the hot metal is intimately mixed with the slag brought about by relading or by charging into a basic oxygen furnace.
Yet another object of this present invention is to propose an improved process for the pretreatment of hot metal for removing silicon, manganese and sulphur all together from molten high carbon and ferrous metal, in a single reaction vessel effectively, economically without hazardous reaction and using inexpensive minerals and chemicals.
Yet another object of the present invention is to propose an apparatus for the pre-treatment of hot metal for the simultaneous removal of silicon, manganese and sulphur from molten high carbon ferrous metal, wherein the quantity of hot metal in the reaction vessel increases thereby, increasing the amount of the hot metal produced.
Still another object of the present invention is to propose an apparatus for carrying out the above mentioned process, and wherein the plunging of magnesium is effected in a controlled manner, which achieves desulphurisation and helps the recovery of iron from the iron oxide present in the slag phase.